The field of the invention pertains to electromagnetic door locks and, in particular, to improved means for sensing adequate locking of the door and means for better assuring release of the door upon collapse of the magnetic field.
The electromagnetic door locks of interest generally comprise oblong magnets constructed of a stacked plurality of "E" shaped magnet iron laminations within a thin case. Laminations are used to reduce the remanence to the lowest level possible. The center pole of the "E" is wrapped with multiple turns of magnet wire. The entire volume within the "E" and containing the wire is potted or sealed with a non-magnetic non-conductive plastic.
U.S. Pat. No. 4,682,801 illustrates such a door lock including its typical location on a door frame and the location of the complementary armature on the door. The armature typically comprises a flat piece of steel that when the door is fully closed preferably contacts or comes into very close proximity to the poles of the "E" to complete a magnetic circuit through the armature. Also illustrated is the electric circuitry for the electromagnet some of the circuitry being positioned at the end of the electromagnet. Typically, the electric circuitry shown is sealed or potted to prevent damage or premature deterioration of the circuit elements. As a result such door locks are extended in length at one end substantially beyond the electromagnet length and are asymmetric in appearance. Illustrative of this design is the Securitron-Magnalock advertisement in the January 1990 issue of Access Control Magazine.
Alternatively, the electric circuitry may be merely enclosed by a removable cover as illustrated in the DynaLock advertisement in the January 1990 issue of Doors & Hardware Magazine. Or the electromagnet case may be further extended at both ends to create a more symmetric appearance.
Residual magnetism drastically affects the time to release after the electric power is disconnected from the electromagnet. For many applications, in particular, emergency door exits, the lock should fully release to less than four pounds residual attractive force within a fraction of a second. To overcome quickly the residual magnetism both mechanical and electrical means have been employed. Plating applied to protect against corrosion of the magnet and armature serves as an "air gap" and therefore substantially counters the remanence of the magnet iron. Mechanically, an elastic grommet or spring is compressed between the armature and the electromagnet when engaged. With the collapse of the electromagnetic field, the grommet or spring urges the armature apart from the electromagnet, releasing the door. The rubber grommet or spring also acts to quiet or silence the lock when the door is closed with the lock energized.
Electrically, capacitor circuits have been employed to reverse the magnetic field upon de-energization of the electromagnet. Reversal of the magnetic field effectively overcomes the residual magnetism. U.S. Pat. No. 3,931,551 and U.S. Pat. No. 4,318,155 disclose such capacitor circuits.
Also electrically, and common in tractive electromagnets other than door locks, for example those employed lifting steel in junkyards, the electric current energizing the coil is simply reversed in polarity for a brief time to accomplish the removal of the residual magnetism. These devices are unacceptable for exit door locks since, for safety, exit doors must be unlocked immediately if there is loss of power to the lock.
With a view toward further improving the electromagnetic door lock technology above by making the release of the armature more positive and effective and to provide a more compact and symmetrical electromagnet, applicant has developed the following improvements.